The primary objectives of this study are: To evaluate the safety of mulitple IM doses of SB 209763 in infants and young children with underlying conditions which predispose them to serious RSV infection. To evaluate the efficacy of mulitple IM doses of SB 209763 in infants and young children as prophylaxis against the development of serious RSV infection; that is, serious enough to require hospitalization. To evaluate whether antibodies against SB 209763 are formed in infants and young children following mulitple IM doses of MAb.